1. Field of the Invention
The present invention relates to a pneumatic tire in which a sipe is formed in a land portion of a tread surface, and is more particularly useful as a studless tire.
2. Description of the Related Art
Conventionally, in a studless tire, a notch called as a sipe is formed in a land portion such as a block, a rib or the like, and a traveling performance on an ice road surface having a low coefficient of friction (hereinafter, refer to as an ice performance) is enhanced based on an edge effect and a drainage effect generated by the sipe. However, since a rubber is incompressible, a motion of the tread surface being in contact with the road surface becomes larger in the case that a load is applied to the tire. Particularly, in the ice road surface, an opening portion of the sipe tends to become narrowed due to a slip of the tread surface, and there is a case that the edge effect and the drainage effect which should be achieved by the sipe are lowered.
In order to solve this problem, in a tire described in Japanese Unexamined Patent Publication No. 2007-8303, a sipe 20 having wide portions 21 to 24 as shown in FIG. 9 is formed in a block 1. The wide portions 21 to 24 are constructed by arranging a concave groove 26 extending in a longitudinal direction of the sipe 20 in sipe wall surfaces 20a and 20b, and in the case that the block 1 is exposed to a load, a deformation generated by the load is absorbed by the wide portions 21 to 24. Accordingly, it is possible to inhibit an opening portion of the sipe 20 from becoming narrower and it is possible to secure an edge effect and a drainage effect.
However, in the sipe structure as mentioned above, since a portion in which a sipe width becomes larger continuously extends along a longitudinal direction, it is known that the block tends to collapse excessively. Such excessive collapse of the block not only weakens the edge effect, but also causes a reduction of a ground contact area of the block, thereby making an adhesive friction effect generated by a softness of the rubber hard to be achieved. In addition, in the above sipe structure, since a sipe width appearing on a tread changes all of a sudden at the wide portion in the process that a wear makes progress, there is fear that a toe and heel wear is generated.
In Japanese Patent No. 4149048, there is described a sipe in which a bottom portion is formed narrowest while making a center portion in a depth direction widest, for reducing a difference of a steering stability between a wet road surface and a dry road surface. However, since the sipe width becomes maximum and even over the whole of the center portion in the depth direction in this sipe, a land portion tends to collapse excessively on the ice road surface, and the sipe width appearing on the tread changes all of a sudden in accordance with the progress of the wear. Accordingly, there can be considered that the problem relating to the ice performance and the toe and heel wear resistance mentioned above is generated.